TY - JOUR
T1 - Why Do Large-Scale Land Surface Models Produce a Low Ratio of Transpiration to Evapotranspiration?
AU - Chang, Li Ling
AU - Dwivedi, Ravindra
AU - Knowles, John F.
AU - Fang, Yuan Hao
AU - Niu, Guo Yue
AU - Pelletier, Jon D.
AU - Rasmussen, Craig
AU - Durcik, Matej
AU - Barron-Gafford, Greg A.
AU - Meixner, Thomas
N1 - Funding Information:
This study was supported by the NASA MAP Program (80NSSC17K0352) and the National Science Foundation (NSF) for the Catalina-Jemez Critical Zone Observatory (NSF-EAR-1331408). We thank Reed Maxwell and other two anonymous reviewers for their constructive suggestions. Data used in this research (e.g., the model inputs in section 2.3) are mostly available through the Critical Zone Observatory (CZO) webpage: http://criticalzone.org/catalina-jemez/data.
Funding Information:
This study was supported by the NASA MAP Program (80NSSC17K0352) and the National Science Foundation (NSF) for the Catalina-Jemez Critical Zone Observatory (NSF-EAR-1331408). We thank Reed Maxwell and other two anonymous reviewers for their constructive suggestions. Data used in this research (e.g., the model inputs in section 2.3) are mostly available through the Critical Zone Observatory (CZO) webpage: http://criticalzone.org/ catalina-jemez/data.
Publisher Copyright:
©2018. American Geophysical Union. All Rights Reserved.
PY - 2018/9/16
Y1 - 2018/9/16
N2 - Most land surface models (LSMs) used in Earth System Models produce a lower ratio of transpiration (T) to evapotranspiration (ET) than field observations, degrading the credibility of Earth System Model-projected ecosystem responses and feedbacks to climate change. To interpret this model deficiency, we conducted a pair of model experiments using a three-dimensional, process-based ecohydrological model in a subhumid, mountainous catchment. One experiment (CTRL) describes lateral water flow, topographic shading, leaf dynamics, and water vapor diffusion in the soil, while the other (LSM like) does not explicitly describe these processes to mimic a conventional LSM using artificially flattened terrain. Averaged over the catchment, CTRL produced a higher T/ET ratio (72%) than LSM like (55%) and agreed better with an independent estimate (79.79 ± 27%) based on rainfall and stream water isotopes. To discern the exact causes, we conducted additional model experiments, each reverting only one process described in CTRL to that of LSM like. These experiments revealed that the enhanced T/ET ratio was mostly caused by lateral water flow and water vapor diffusion within the soil. In particular, terrain-driven lateral water flows spread out soil moisture to a wider range along hillslopes with an optimum subrange from the middle to upper slopes, where evaporation (E) was more suppressed by the drier surface than T due to plant uptake of deep soil water, thereby enhancing T/ET. A more elaborate representation of water vapor diffusion from a dynamically changing evaporating surface to the height of the surface roughness length reduced E and increased the T/ET ratio.
AB - Most land surface models (LSMs) used in Earth System Models produce a lower ratio of transpiration (T) to evapotranspiration (ET) than field observations, degrading the credibility of Earth System Model-projected ecosystem responses and feedbacks to climate change. To interpret this model deficiency, we conducted a pair of model experiments using a three-dimensional, process-based ecohydrological model in a subhumid, mountainous catchment. One experiment (CTRL) describes lateral water flow, topographic shading, leaf dynamics, and water vapor diffusion in the soil, while the other (LSM like) does not explicitly describe these processes to mimic a conventional LSM using artificially flattened terrain. Averaged over the catchment, CTRL produced a higher T/ET ratio (72%) than LSM like (55%) and agreed better with an independent estimate (79.79 ± 27%) based on rainfall and stream water isotopes. To discern the exact causes, we conducted additional model experiments, each reverting only one process described in CTRL to that of LSM like. These experiments revealed that the enhanced T/ET ratio was mostly caused by lateral water flow and water vapor diffusion within the soil. In particular, terrain-driven lateral water flows spread out soil moisture to a wider range along hillslopes with an optimum subrange from the middle to upper slopes, where evaporation (E) was more suppressed by the drier surface than T due to plant uptake of deep soil water, thereby enhancing T/ET. A more elaborate representation of water vapor diffusion from a dynamically changing evaporating surface to the height of the surface roughness length reduced E and increased the T/ET ratio.
KW - ET partitioning
KW - complex mountain terrain
KW - land surface models (LSMs)
KW - lateral surface and subsurface flows
KW - soil surface evaporation
KW - three-dimensional process-based ecohydrological model
UR - http://www.scopus.com/inward/record.url?scp=85052941321&partnerID=8YFLogxK
UR - http://www.scopus.com/inward/citedby.url?scp=85052941321&partnerID=8YFLogxK
U2 - 10.1029/2018JD029159
DO - 10.1029/2018JD029159
M3 - Article
AN - SCOPUS:85052941321
SN - 2169-897X
VL - 123
SP - 9109
EP - 9130
JO - Journal of Geophysical Research Atmospheres
JF - Journal of Geophysical Research Atmospheres
IS - 17
ER -